The following background is for convenience of those skilled in the art and for incorporating the listed citations by reference. The following background information is not an assertion that a search has been made, or that the following citations are analogous art, or that any of the following citations are pertinent or the only pertinent art that exists, or that any of the following citations are prior art.
The continued introduction of very high I/O and very high density surface mount components especially 0.2-0.4 mm gull wing leaded components, 40 mil ball grid array BGA modules, as well as the direct connection flip chips to circuit boards, has resulted in a need for very high density conductor fan out at these components. At the same time, decrease in the size of plated through holes PTHs which interconnect between wiring layers, has not kept up with these requirements for fan out. PTHs require substantial surface area which can not be easily reduced because seeding and plating require circulation of fluids in the holes. Reducing the size of connections between wiring layers has become critical for continued increase in circuit board density.
For such high density surface mount components, solder volummes are a critical process variable, but when components are attached to PTHs even those filled with solder, the solder volume between the terminals of the components and the PTHs can not easily be controlled.
Those skilled in the art are directed to the following references. U.S. Pat. No. 4,967,314 to Higgins, III suggests filling via interconnect holes with a conductive epoxy. U.S. Pat. No. 3,163,588 to Shortt suggests stripable frisket, seeding and electroplating. Face Protection of Printed Circuit Boards by McDermott in IBM Technical Disclosure Bulletin Vol. 11 No. 7 December 1968 describes peelable coverings and pressing resin into plated through holes. Printed Circuit Base by Marshall in IBM TDB Vol. 10, No. 5, October 1967, describes a sensitizing material. U.S. Pat. No. 4,590,539 to Sanjana discloses epoxies, fillers, curing agents, and catalysts. U.S. Pat. No. 4,791,248 to Oldenettel suggests peel apart coverings, filling holes with resin, and planing off resin nubs. U.S. Pat. No. 4,893,404 to Shirahata discloses buried vias and electroconductive organic based paste. U.S. Pat. No. 4,964,948 to Reed suggests methods for seeding a substrate for electroplating. U.S. Pat. Nos. 4,991,060 and 5,028,743 to Kawakami suggests filling through holes with electroconductive paste and buried vias. U.S. Pat. No. 5,065,227 to Frankeny suggests electrically conductive paste filling a via hole. U.S. Pat. No. 5,243,142 to Ishikawa discloses hole fill. U.S. Pat. No. 5,271,150 to Isasaka discloses manufacturing methods for multi-layer ceramic substrates including filling holes punched in green sheets with conductive paste. U.S. Pat. No. 5,319,159 to Watanabe suggests method of manufacturing a double sided printed wiring boards with resin filled PTHs. Japanese application 2-045998 suggests filling through holes with electroconductive thermosetting paste. Japanese application 2-184626 to Honda suggests using a novolac epoxy resin such as cresol novolac epoxy resin for a circuit board. U.S. Pat. No. 5,346,750 to Hatakeyama suggests a method to prevent bleed out of paste from a filled via. U.S. Pat. Nos. 4,354,895 to Ellis, 5,057,372 to Imfeld, and 5,262,247 to Kajiwara suggests a metal foil with a peel apart protective layer. U.S. Pat. No. 5,200,026 to Okabe and 5,266,446 to Chang suggest processes for forming thin film structures on substrates. U.S. Pat. Nos. 4,940,651 to Brown, 5,026,624 to Day, 5,070,002 to Leech, 5,300,402 to Card, 5,427,895 to Magnuson, and 5,439,779 to Day discuss photoresists. U.S. Pat. Nos. 4,127,699 to Aumiller, 4,210,704 to Chandross, 4,731,503 to Kitanishi, 4,747,968 to Gilleo, 4,822,523 to Prud'Homme, 4,880,570 to Sanborn, 4,904,414 to Peltz, 4,999,136 to Su, 5,082,595 to Glackin, 5,220,724 to Gerstner, and 5,463,190 to Carson suggest various electrically isotropically conductive organic materials. New Avenues for Microvias in Electronic Engineering Times, Mar. 18, 1996 p. 68 reports that Prolinx Labs Corp of San Jose Calif. has developed an additive technology for blind and buried vias filled with conductive material. The proceeding citations are hereby incorporated in whole by reference.